This invention relates generally to in-line electron guns for use in three beam cathode ray tubes and especially to a novel in-line electron gun of the SB (symmetrical outer beam) type that has a main lens that is configured to reduce spherical aberration.
In-line electron guns for cathode ray tubes are well known in the art. U.S. Pat. No. 5,170,101, assigned to Zenith Electronics Corporation, describes an in-line SB type electron gun for use in a high resolution tri-beam color cathode ray tube that has a self convergent yoke, as do most modern color cathode ray tubes. As is well known, such yokes undesirably introduce beam shape distortions which may be compensated by various electron lens configurations, one such being a dynamic lens, often referred to as a Dynamic Quadropole. Other types of distortion are also introduced to the electron beams by the electron lenses of the gun.
A major problem is spherical aberration that results when all portions of the electron beam do not focus at the same point because of the inherent increase in focussing power with radial distance from the optical center of the lens. The effect is that of halo about the electron spot, which detracts from resolution. This type of distortion may be compensated by making the lens appear large in comparison with the size of the electron beam.
The present invention reduces spherical aberration in the electron gun by enlarging and weakening the apparent aperture of the main lens. This is accomplished by enlarging the apertures in the G3 and G4 electrodes and by separating them by a greater than normal amount.